Authors:

Michael Krieg(University of Hawaii)

Kamran Mohseni(University of Florida)

Jetting propulsion has historically been considered inefficient, as the rate
of momentum transfer for a continuous jet scales with the velocity squared;
whereas, the rate of kinetic energy scales with the velocity cubed. For
steady jets, efficiency decreases with the ratio of jet velocity to vehicle
velocity. Several animals propel themselves with high velocity jets, but
none jet continuously. They pause between jetting to refill, and expel the
next jet starting from rest resulting in a leading vortex ring. Vortex ring
formation induces a converging radial velocity increasing hydrodynamic
impulse and increasing cavity pressure. Also, fluid acceleration generates
propulsion without significant wake energy. This study validates improved
propulsive efficiency on a freely swimming autonomous underwater vehicle
(AUV). We have developed AUVs that use such thrusters for maneuvering, and
previously validated propulsive efficiency measurement using motion capture
position data and motor frequency data. But in a maneuvering configuration
the thrusters have lower propulsive efficiency due to losses from vehicle
drag. With a streamlined AUV, we demonstrate that propulsive efficiency of
unsteady jetting rivals that of unducted propellers, and is nearly double
the efficiency in a maneuvering configuration.